This invention relates to heat exchangers having nucleation and boiling surfaces for boiling liquids.
It is known that the nucleation and boiling characteristics of a surface can be modified by altering the physical texture of the surface. The process of boiling requires the nucleation of minute bubbles in the liquid which then grow to a size sufficient for them to be released from the liquid. It is preferred that there is a large number of nucleation sites in a liquid so that many reasonably small bubbles can form, rather than a few large ones. When liquefied gases are evaporated in heat exchangers it is desired that the gases nucleate and form bubbles as easily as possible. It has been proposed, therefore, to roughen the surface of the inside of a heat exchanger to enhance the nucleation and boiling characteristics of the heat exchanger surfaces. Such roughening is somewhat complex to arrange and hence expensive. There is also a problem in that the formation of a minute bubble at one nucleation site on a surface results in heat being drawn to that site from the surrounding area to evaporate liquid into the forming bubble. This means that the entire surface area of the surface is never in complete production of small bubbles, as sites are continuously formed and influence their surrounding areas to prevent further nucleation in a very localised region. To date the methods of improving the nucleation and boiling characteristics of surfaces have in the main related to methods and processes for texturising the surfaces.
Additionally there have been proposals to provide nucleate boiling surfaces on the heated surface of a heat exchanger by preparing closely spaced grooves in the metal wall and then deforming the outer ridges over the ends of the grooves so that cavities are provided with narrow exits from the re-entrant cavities. Essentially this method is used on tubular heat exchangers because of the relative ease with which the grooves may be formed in circular tube walls. Such a heat exchanger is described in U.S. Pat. No. 3,454,081.
In UK Patent Specification No. 1,328,919 there is a proposal to provide stamped metal foil promoters which are attached to a liquid boiler wall, the metal foil having a three-dimensional array of pyramid shaped elements which are pierced on their top and which are bonded to a base surface. It has also been proposed to use loosely fitting wire or non-metallic cord wrapped in the space between the fins of an integral finned tube, see for example U.S. Pat. No. 3,521,708.